1. Technical Field
The invention relates to the compression of digital image data in a computer environment. More particularly, the invention relates to the compression of digital image data during the dithering process in a computer environment.
2. Description of the Prior Art
Digital image rendering involves comparing an array of pixel values of a source image and a threshold array to generate a new array of pixels values, called the dithered image. The pixels in the dithered image usually has less information (bits per pixel) than the pixels in the source image.
The source image must go through several stages such as storage and halftoning before printing. The source image is normally compressed before it is stored on a computer system. This is because the raw data requires a large amount of memory. For example, an 8.5".times.11" letter-sized, full color image normally takes approximately 64 MB to 70 MB of memory (on the hard disk or RAM), uncompressed. Image data are lost during the compression stages whenever a lossy compression technique, e.g., Joint Photographic Experts Group (JPEG), is used to compress the source image.
Many printing devices are not capable of reproducing gray scale images because they are bi-level. As a result, the binary representation of a gray scale image is a necessity in a wide range of applications such as laser printers, facsimile machines, lithography, liquid crystal displays, and plasma panels. Gray scale and color images are typically converted to binary images using halftone techniques. A typical example is halftoning, or dithering, an eight-bit/pixel source image to generate a one bit/pixel "bitmap" for laser printer output.
Halftone generally refers to the process of generating continuous variations of intensity on a printed page using arrays of discrete pixels whose intensity is binary. Halftoning renders the illusion of various shades of gray by using binary levels, black and white, and can be implemented either digitally (laser printers) or optically (newspaper printing).
After an image goes through the halftone process, the halftone image is compressed for storage on the computer system because of memory demands. Compressing a halftone image is usually a challenging task because the pixel values in the dithered image may appear random to a software program and a high compression ratio can be difficult to achieve.
In addition, the threshold array process is considered the major contributor to the loss of information as well as adding noise which makes compression difficult.
Halftoning, or dithering, techniques using dither matrices are described in U.S. Pat. No. 4,760,460 issued to Shimotohno on Jul. 26, 1988, which teaches a method for transmitting a halftone image using a dithering method which divides an image into a plurality of regions corresponding to the dither matrix used. Average density values are calculated to determine addresses for encoding, compression, and transmission.
Typical halftoning techniques such as using blue noise masks are described in U.S. Pat. Nos. 5,708,518 and 5,323,247 issued to Parker et al on Jan. 13, 1998, which teaches a method and system for rendering a halftone image of a gray scale image by utilizing a pixel-by-pixel comparison of the gray scale image against a blue noise mask.
Methods for compressing computer images are described in several publications. U.S. Pat. No. 5,542,031 issued to Douglass et al on Jul. 30, 1996, describes a method for processing computerized data in Postscript format through an interpreter and rasterizer utilizing parallel processing.
U.S. Pat. No. 5,522,898 issued to Deschuytere on Sep. 3, 1996, describes a method using lossy and lossless compression in a raster image processor which generates a compressed bitmap, bitmask, and contone map of an image and reconstructing said image on the fly using said bitmap, bitmask, and contone map.
U.S. Pat. No. 5,359,430 issued to Zhang on Oct. 25, 1994, describes a method for converting continuous-tone images into a first and second halftone image and error image. Scattered pixels in said error image are removed and said error image is compressed and sent to a receiver which reverses the process of creating said error image.
U.S. Pat. No. 5,457,772 issued to Shannon on Oct. 10, 1995, and U.S. Pat. No. 5,548,689 issued to Poppenga et al on Aug. 20, 1996, teach a method for converting source color image data to a rasterized monochrome bit map which involves a conversion cycle for converting source pixels to gray scale values.
However, these methods do not address the problem of image data loss during the storage and halftoning processes.
It would be advantageous to provide a dithered image compression system that limits the amount of image data lost during the storage and halftone processes, yet preserves the visual appearance of the original source image result, thus creating a more efficient image file. It would further be advantageous to provide a dithered image compression system that takes advantage of knowing the amount of data lost during the storage and halftone stages, thereby giving the user the ability to adjust to the desired image quality and compression ratio.